In order for T cells to respond to foreign proteins, two signals must be provided by antigen-presenting cells (APCs) to resting T lymphocytes (Jenkins, M. and Schwartz, R. (1987) J. Exp. Med 165:302-319; Mueller, D. L. et al. (1990) J. Immunol. 144:3701-3709). The first signal, which confers specificity to the immune response, is transduced via the T cell receptor (TCR) following recognition of foreign antigenic peptide presented in the context of the major histocompatibility complex (MHC). The second signal, termed costimulation, induces T cells to proliferate and become functional (Lenschow et al. (1996) Annu. Rev. Immunol. 14:233). Costimulation is neither antigen-specific, nor MHC restricted and is thought to be provided by one or more distinct cell surface molecules expressed by APCs (Jenkins, M. K. et al. (1988) J. Immunol. 140:3324-3330; Linsley, P. S. et al. (1991) J. Exp. Med 173:721-730; Gimmi, C. D. et al. (1991) Proc. Natl. Acad. Sci. USA 88:6575-6579; Young, J. W. et al. (1992) J. Clin. Invest 90:229-237; Koulova, L. et al. (1991) J. Exp. Med. 173:759-762; Reiser, H. et al. (1992) Proc. Natl. Acad. Sci. USA 89:271-275; van-Seventer, G. A. et al. (1990) J. Immunol. 144:4579-4586; LaSalle, J. M. et al. (1991) J. Immunol. 147:774-80; Dustin, M. I. et al. (1989) J. Exp. Med 169:503; Armitage, R. J. et al. (1992) Nature 357:80-82; Liu, Y. et al. (1992) J. Exp. Med. 175:437-445).
The CD80 (B7-1) and CD86 (B7-2) proteins, expressed on APCs, are critical costimulatory molecules (Freeman et al. (1991) J. Exp. Med. 174:625; Freeman et al. (1989) J. Immunol. 143:2714; Azuma et al. (1993) Nature 366:76; Freeman et al. (1993) Science 262:909). B7-2 appears to play a predominant role during primary immune responses, while B7-1, which is upregulated later in the course of an immune response, may be important in prolonging primary T cell responses or costimulating secondary T cell responses (Bluestone (1995) Immunity 2:555).
One ligand to which B7-1 and B7-2 bind, CD28, is constitutively expressed on resting T cells and increases in expression after activation. After signaling through the T cell receptor, ligation of CD28 and transduction of a costimulatory signal induces T cells to proliferate and secrete IL-2 (Linsley, P. S. et al. (1991) J. Exp. Med 173:721-730; Gimmi, C. D. et al. (1991) Proc. Natl. Acad. Sci. USA 88:6575-6579; June, C. H. et al. (1990) Immunol. Today 11:211-6; Harding, F. A. et al. (1992) Nature 356:607-609). A second ligand, termed CTLA4 (CD152) is homologous to CD28 but is not expressed on resting T cells and appears following T cell activation (Brunet, J. F. et al. (1987) Nature 328:267-270). CTLA4 appears to be critical in negative regulation of T cell responses (Waterhouse et al. (1995) Science 270:985). Blockade of CTLA4 has been found to remove inhibitory signals, while aggregation of CTLA4 has been found to provide inhibitory signals that downregulate T cell responses (Allison and Krummel (1995) Science 270:932). The B7 molecules have a higher affinity for CTLA4 than for CD28 (Linsley, P. S. et al. (1991) J. Exp. Med 174:561-569) and B7-1 and B7-2 have been found to bind to distinct regions of the CTLA4 molecule and have different kinetics of binding to CTLA4 (Linsley et al. (1994) Immunity 1:793).
In the past, reports of the existence of additional members of the B7 costimulatory family have been controversial. The antibody BB-1, appeared to recognize a subset of cells greater than either B7-1 or B7-2 positive cells, arguing for the existence of another B7-family member, B7-3. The identity of B7-3 had been in part thought to be answered by expression cloning of T-cell receptor invariant chain using the BB-1 antibody. Although invariant chain is not related to the B7 family, this molecule facilitated a low degree of costimulation when assessed by T cell proliferation assays.
Very recently, a novel surface receptor termed ICOS was described which had sequence identity with CD28 (24%) and CTLA4 (17%) (Hutloff et al. (1999) Nature 397:263; WO 98/38216). Unlike CD28, ICOS was shown to be upregulated on stimulated T cells and caused the secretion of a panel of cytokines distinct from those mediated by CD28 costimulation (Hutloff et al. (1999) Nature 397:263).
The importance of the B7:CD28/CTLA4 costimulatory pathway has been demonstrated in vitro and in several in vivo model systems. Blockade of this costimulatory pathway results in the development of antigen specific tolerance in murine and human systems (Harding, F. A. et al. (1992) Nature 356:607-609; Lenschow, D. J. et al. (1992) Science 257:789-792; Turka, L. A. et al. (1992) Proc. Natl. Acad. Sci. USA 89:11102-11105; Gimmi, C. D. et al. (1993) Proc. Natl. Acad. Sci. USA 90:6586-6590; Boussiotis, V. et al. (1993) J. Exp. Med. 178:1753-1763). Conversely, expression of B7 by B7 negative murine tumor cells induces T-cell mediated specific immunity accompanied by tumor rejection and long lasting protection to tumor challenge (Chen, L. et al. (1992) Cell 71:1093-1102; Townsend, S. E. and Allison, J. P. (1993) Science 259:368-370; Baskar, S. et al. (1993) Proc. Natl. Acad. Sci. USA 90:5687-5690.). Therefore, manipulation of the costimulatory pathways offers great potential to stimulate or suppress immune responses in humans.